1. Field of the Invention
This invention relates generally to a current source, and, more particularly, to a low-noise, current source used, for example, in a voltage controlled oscillator.
2. Description of the Related Art
In the field of wireless telecommunications, noise problems are a significant problem. For example, in cordless telephone sets, a handset and base communicate over a radio link instead of through a wire. Noise may reduce the useable range, or more problematic, even eliminate proper operation by interfering with the radio link between the handset and the base of the telephone set.
Some noise is environmental in nature. That is, some noise comes from the environment in which the telephone set is located. For example, other electronic equipment operating near the telephone set can produce EMI (electromagnetic interference) that causes noise to be impressed onto the circuitry of the telephone set. Shielding the telephone set can reduce some environmental noise, however, in applications such as cordless telephones, shielding may not be entirely effective. In fact, cordless telephones have been recently designed to operate in a frequency range (.about.900 MHz) that should normally be outside the range of noise produced by many home appliances. That is, the radio signals delivered between the handset and the telephone base are carried by a 900 MHz electromagnetic signal. Most home appliances do not produce EMI in this frequency range.
Another type of noise that can impair voice signal clarity arises from inside the telephone set. That is, the telephone set may produce noise internally that is impressed on the radio signal. Two examples are device noise and switching noise. Oscillators are commonly used to produce the .about.900 MHz carrier signal used in cordless telephones. Any noise introduced by the oscillator will ultimately be delivered over the radio link, interfering with the proper operation.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.